Hydrocephalus is a brain condition in which cerebrospinal fluid accumulates at abnormally high pressure in ventricles or chambers within the brain. The ventricles expand in response to the pressure exerted by the fluid, and surrounding brain tissue is compressed between the ventricles and the skull. Hydrocephalus usually occurs in babies or young children, and, if unchecked, results in brain damage, enlargement and deformation of the head, and eventual death.
Modern medical methods are effective in arresting many cases of hydrocephalus, but it is often desirable to monitor pressure of the cerebrospinal fluid over an extended period to detect relapse and to determine long-range effectiveness of treatment. In the past, this measurement has been made by surgically implanting a miniature but generally conventional transducer such as a strain-gage-bridge pressure pickup. This technique requires that wiring be conducted from the implanted transducer to external instrumentation which provides excitation voltage to the bridge and detects bridge-unbalance voltage signals indicative of pressure. Alternatively, non-electrical manometric measurement methods may be used, but these techniques require installation of a conduit extending from the interior of the brain ventricle through the skull and scalp to external measurement equipment.
The primary disadvantage of these known techniques is that they involve conducting an electrical cable or fluid tube through the skull and scalp to enable direct electrical or mechanical connection between the interior of the brain ventricle and external equipment. This connection is disturbing and uncomfortable for the patient, and the danger of infection of tissue surrounding the cable or tube (and the risk of infection spread resulting in meningitis, ventriculitis, brain abscess or septicemia) requires constant supervision and usually full-time hospitalization of the patient. There is accordingly a need for a measuring device which does not require direct electrical or mechanical connection from the brain to external equipment, and which permits the patient to be ambulatory after the device is installed. pg,3
Connection-free implantable transducers have been previously proposed, and they typically function by external detection of the resonant frequency of a resonant circuit in the implanted device. For example, the prior art includes a biological pressure transducer for sensing pressure in the gastrointestinal tract and having a resonant circuit with a pressure-controlled inductor. Wireless systems are also used for sensing EEG or ECG voltages, the implantable part of the system using an electrically variable capacitor in a resonant circuit. A wireless resonantcircuit transducer has also been used for measuring intraocular pressure, the transducer using a pair of variably spaced Archimedean-spiral coils mounted on pressure-sensitive diaphragms.
The transducer of this invention operates in wireless fashion similar to the instruments described above, but provides improved performance and lower drift in implantation applications involving placement in body cavities such as brain ventricles or heart chambers where only a very small transducer can be tolerated. The transducer and is disclosed below in a specific form suitable for intracranial implantation to monitor pressure of cerebrospinal fluid in a brain ventricle. This form is also suitable for mounting on hydrocephalus shunt apparatus as often used in treating and controlling this disease.
Our transducer is, however, also suitable for implantation elsewhere in the body, and is believed to be useful in any application where a very small, implantable and wireless device is needed to measure fluid or tissue pressure. For example, the transducer is believed useful for either short- or long-term monitoring of abnormal intracranial pressure in head-injury patients, or for post-surgical monitoring of brain-tumor victims to detect possible recurrence of the tumor. When such monitoring is no longer needed, the implanted transducer is removed by a simple re-opening and closure of the overlying scalp tissue.